Merge branch 'fix/wrong-license-in-build-system-file' into 'master'

Fix wrong license in build system doc

See merge request !1508

doc/buildsystem/dune-common.rst.license

-SPDX-FileCopyrightInfo: Copyright (c) 2017, Dominic Kempf, Steffen Müthing
-SPDX-License-Identifier: BSD-3-Clause
+SPDX-FileCopyrightInfo: Copyright © DUNE Project contributors, see file LICENSE.md in module root
+SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception
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